Savings from Fewer Engines

[Peter47]

I asked the same roughly the same question eight years ago - why are two engine aircraft more fuel efficient than four.

Only possible answers:

- The engines used on twins have a lower specific fuel consumption.

- Twins are lighter than quads (the A330 is certainly lighter than the A340 although it has a slightly lower range). I don't know if engine weight is a factor.

- Some aerodynamic factor - which would surprise me.

Twin jets will probably have a better power/weight ratio in order to cope with an engine out t/o (the A340 is notoriously underpowered as were early 747s) but if that is a factor just put more powerful jets on the 340.

The 77W has the same payload as the 744 but burns 20 - 25% less fuel but this may be down to design advances over the years. The 146 had four engines because they needed four of the best type available. If you put four engines of half the power with the same sfc on the 777 would you burn the same amount of fuel? I presume so.

Checking my old (and probably well out of date) coursework notes I see that a 343 of a similar vintage to a 333 burns about 12.5% less fuel. MTOW is 275t v 230t, OWE 170.3t v 124.3t so you would expect it to burn less. However range is over 4,000km greater. (Newer 330s will doubtless go further).

The 77W is more efficient than the 748 on a per seat basis but that may be because it is based on a 45 year old design.

I can't comment on how having fewer engines reduces maintenance & capital cost against the need to factor in greater reliability & would welcome anyone's comments, but evidence from the real world suggest that twins win hands down.

I presume that the reason that 757s has such low seat costs on transatlantic routes is their low weight (offset by very limited range and negligible cargo capacity).